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Bud Burst in Trees - Spring is here!

You may have noticed some of your trees beginning to awaken after their winter slumber. Bud burst can be one of the first signs that a vulnerable tree has made it through the winter, but it is also a delicately balanced process which has evolved over thousands of years to work in harmony with the weather. So how does it happen?

When the tree went into dormancy in the autumn, a corky layer formed at the base of the tree which restricted sap flow. This initially drained the green chlorophyll and eventually caused the leaf to die and fall. The winter resting bud, which is packed full of dividing cells, became protected by a corky, woody substance, often in the form of scales. The tree did this to reduce water loss from leaf transpiration because it would not be able to replace that water from its roots over the cold, winter months.

As the temperature has been increasing over the past month, enzymes within the tree have become active again and begun to break down this corky layer. When the layer is thin enough, those young, dividing cells can burst through and begin to grow a leaf. As we hit a prolonged colder spell over the past week, that corky layer thickens up again, protecting the delicate cells underneath.

Timing of bud burst is crucial to the tree. If it occurs too early, a late spring frost can easily damage new, fleshy growth which will result in stunted growth and can kill an apically dominant shoot. If a secondary shoot has to become dominant, then this normally causes the physical structure of the tree to be weaker. If bud burst occurs too late, then the tree misses out on the opportunity of an extended growing season.

Have you noticed that all trees break dormancy at different times? That is because each tree has evolved to come into bud in time for spring at its place of origin, getting maximum use of the growing season while avoiding the spring frosts. So trees of a southerly origin will break dormancy before those from the north. Take a look around your local trees - can you guess the latitude of their origins from the timing of their bud burst?